1. Field of the Invention
The invention is directed to an improved brushless DC machine, and to a return ring packet, a toothed rim packet, and a top ice of the DC machine, as well as a method for producing the brushless DC machine.
2. Description of the Prior Art
Brushless DC machines (BLDC direct current machines), electrically commutated motors (EC motors), and switched reluctance motors (SR motors) are known. They have excellent electrical and mechanical properties, but have comparatively high production costs compared to DC machines with mechanical commutation (DC direct current machines). For this reason, EC DC machines have until now not found any significant use in low cost applications, for instance as in power tools or automotive applications. Since an EC DC machine for commutation fundamentally requires a semiconductor bridge circuit along with the associated control electronics, it has a strategic disadvantage compared to a DC direct current machine that does not require this expenditure.
In the prior art, for controlling DC machines (direction of rotation, rotary speed, torque), electronics have meanwhile come to be used increasingly often, so that the additional expense required for controlling an EC DC machine is becoming less and less significant.
However, the production methods for the known EC motors are expensive and complicated. The EC motor includes a stator constructed of stamped laminations. The stator serves to receive one or more stator windings, with which the performance specifications of the motor are essentially defined. For the windings, slots are for instance embodied in the stator. A rotor disposed on a motor shaft has a plurality of permanent magnets, which are disposed opposite the stator, separated by an air gap. The stator windings can be excited in succession in such a way that a rotary field that sets the rotor into rotation is generated.
The following winding methods are known in the prior art:
In a first method, a pull-in winding is produced; a winding for a stator is wound onto a core and is then pulled through gaps between teeth into slots of a stator. A disadvantage of this is that this method is relatively complicated and expensive and involves long cycle times. Furthermore, a wide tooth gap is necessary, which in turn has an adverse effect on magnetic properties of the motor thus produced.
In an alternative production method, a single-tooth or double-tooth winding is made. Here the stator is divided into two parts, a toothed rim and a return ring. So that the teeth of the toothed rim will not fall apart, they must remain joined together toward the rotor; that is, toward the rotor, the teeth are not separated by a gap. The connection of the teeth can also be provided toward the rotor only by a number of stamped laminations that suffices for the mechanical strength. A lamination packet of this kind, such as the stator of a conventional DC direct current machine, can be wound simply and quickly in one work step. As an example of a winding method, a single-tooth winding on a coil holder is known, in which after the winding is applied the holder is thrust from outside onto the teeth of the toothed rim. In the classical double-tooth winding method, the full number of windings is applied to one tooth, while the neighboring tooth remains free. However, precisely with short stator lengths, for instance of less 20 mm, the inoperative proportion of the wire of the winding heads is clearly measurable, so that the single-tooth winding method is to be preferred. In it, half of the numbers of windings are each applied to two adjacent teeth, with a contrary winding direction.
Following the winding process, the return ring packet, also made by stamping, is pressed by positive engagement onto the wound toothed rim packet. A disadvantage is that because of the pressing process, major mechanical stresses are created in the toothed rim packet that have to be absorbed by the tooth connections on the rotor side. These tooth connections can therefore not be removed again after the manufacture of the stator, which in turn has an adverse effect on the magnetic properties of the stator.
Moreover, because of the pressing operation, electrical connections among the individual stamped laminations of the packet are created at the contact point, leading to increased losses from eddy currents.